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LKMM pull request for v6.1

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This pull request includes several documentation updates.
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Merge tag 'lkmm.2022.09.30a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull LKMM (Linux Kernel Memory Model) updates from Paul McKenney:
 "Several documentation updates"

* tag 'lkmm.2022.09.30a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt
  docs/memory-barriers.txt: Fixup long lines
  docs/memory-barriers.txt: Fix confusing name of 'data dependency barrier'
This commit is contained in:
Linus Torvalds 2022-10-03 10:06:15 -07:00
commit b8fb65e1d3
2 changed files with 122 additions and 92 deletions
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177
Documentation/memory-barriers.txt
View file

@ -52,7 +52,7 @@ CONTENTS
- Varieties of memory barrier. - Varieties of memory barrier.
- What may not be assumed about memory barriers? - What may not be assumed about memory barriers?
- Data dependency barriers (historical). - Address-dependency barriers (historical).
- Control dependencies. - Control dependencies.
- SMP barrier pairing. - SMP barrier pairing.
- Examples of memory barrier sequences. - Examples of memory barrier sequences.
@ -187,9 +187,9 @@ As a further example, consider this sequence of events:
B = 4; Q = P; B = 4; Q = P;
P = &B; D = *Q; P = &B; D = *Q;
There is an obvious data dependency here, as the value loaded into D depends on There is an obvious address dependency here, as the value loaded into D depends
the address retrieved from P by CPU 2. At the end of the sequence, any of the on the address retrieved from P by CPU 2. At the end of the sequence, any of
following results are possible: the following results are possible:
(Q == &A) and (D == 1) (Q == &A) and (D == 1)
(Q == &B) and (D == 2) (Q == &B) and (D == 2)
@ -391,58 +391,62 @@ Memory barriers come in four basic varieties:
memory system as time progresses. All stores _before_ a write barrier memory system as time progresses. All stores _before_ a write barrier
will occur _before_ all the stores after the write barrier. will occur _before_ all the stores after the write barrier.
[!] Note that write barriers should normally be paired with read or data [!] Note that write barriers should normally be paired with read or
dependency barriers; see the "SMP barrier pairing" subsection. address-dependency barriers; see the "SMP barrier pairing" subsection.
(2) Data dependency barriers. (2) Address-dependency barriers (historical).
A data dependency barrier is a weaker form of read barrier. In the case An address-dependency barrier is a weaker form of read barrier. In the
where two loads are performed such that the second depends on the result case where two loads are performed such that the second depends on the
of the first (eg: the first load retrieves the address to which the second result of the first (eg: the first load retrieves the address to which
load will be directed), a data dependency barrier would be required to the second load will be directed), an address-dependency barrier would
make sure that the target of the second load is updated after the address be required to make sure that the target of the second load is updated
obtained by the first load is accessed. after the address obtained by the first load is accessed.
A data dependency barrier is a partial ordering on interdependent loads An address-dependency barrier is a partial ordering on interdependent
only; it is not required to have any effect on stores, independent loads loads only; it is not required to have any effect on stores, independent
or overlapping loads. loads or overlapping loads.
As mentioned in (1), the other CPUs in the system can be viewed as As mentioned in (1), the other CPUs in the system can be viewed as
committing sequences of stores to the memory system that the CPU being committing sequences of stores to the memory system that the CPU being
considered can then perceive. A data dependency barrier issued by the CPU considered can then perceive. An address-dependency barrier issued by
under consideration guarantees that for any load preceding it, if that the CPU under consideration guarantees that for any load preceding it,
load touches one of a sequence of stores from another CPU, then by the if that load touches one of a sequence of stores from another CPU, then
time the barrier completes, the effects of all the stores prior to that by the time the barrier completes, the effects of all the stores prior to
touched by the load will be perceptible to any loads issued after the data that touched by the load will be perceptible to any loads issued after
dependency barrier. the address-dependency barrier.
See the "Examples of memory barrier sequences" subsection for diagrams See the "Examples of memory barrier sequences" subsection for diagrams
showing the ordering constraints. showing the ordering constraints.
[!] Note that the first load really has to have a _data_ dependency and [!] Note that the first load really has to have an _address_ dependency and
not a control dependency. If the address for the second load is dependent not a control dependency. If the address for the second load is dependent
on the first load, but the dependency is through a conditional rather than on the first load, but the dependency is through a conditional rather than
actually loading the address itself, then it's a _control_ dependency and actually loading the address itself, then it's a _control_ dependency and
a full read barrier or better is required. See the "Control dependencies" a full read barrier or better is required. See the "Control dependencies"
subsection for more information. subsection for more information.
[!] Note that data dependency barriers should normally be paired with [!] Note that address-dependency barriers should normally be paired with
write barriers; see the "SMP barrier pairing" subsection. write barriers; see the "SMP barrier pairing" subsection.
[!] Kernel release v5.9 removed kernel APIs for explicit address-
dependency barriers. Nowadays, APIs for marking loads from shared
variables such as READ_ONCE() and rcu_dereference() provide implicit
address-dependency barriers.
(3) Read (or load) memory barriers. (3) Read (or load) memory barriers.
A read barrier is a data dependency barrier plus a guarantee that all the A read barrier is an address-dependency barrier plus a guarantee that all
LOAD operations specified before the barrier will appear to happen before the LOAD operations specified before the barrier will appear to happen
all the LOAD operations specified after the barrier with respect to the before all the LOAD operations specified after the barrier with respect to
other components of the system. the other components of the system.
A read barrier is a partial ordering on loads only; it is not required to A read barrier is a partial ordering on loads only; it is not required to
have any effect on stores. have any effect on stores.
Read memory barriers imply data dependency barriers, and so can substitute Read memory barriers imply address-dependency barriers, and so can
for them. substitute for them.
[!] Note that read barriers should normally be paired with write barriers; [!] Note that read barriers should normally be paired with write barriers;
see the "SMP barrier pairing" subsection. see the "SMP barrier pairing" subsection.
@ -550,17 +554,21 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
Documentation/core-api/dma-api.rst Documentation/core-api/dma-api.rst
DATA DEPENDENCY BARRIERS (HISTORICAL) ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
------------------------------------- ----------------------------------------
As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
DEC Alpha, which means that about the only people who need to pay attention DEC Alpha, which means that about the only people who need to pay attention
to this section are those working on DEC Alpha architecture-specific code to this section are those working on DEC Alpha architecture-specific code
and those working on READ_ONCE() itself. For those who need it, and for and those working on READ_ONCE() itself. For those who need it, and for
those who are interested in the history, here is the story of those who are interested in the history, here is the story of
data-dependency barriers. address-dependency barriers.
The usage requirements of data dependency barriers are a little subtle, and [!] While address dependencies are observed in both load-to-load and
load-to-store relations, address-dependency barriers are not necessary
for load-to-store situations.
The requirement of address-dependency barriers is a little subtle, and
it's not always obvious that they're needed. To illustrate, consider the it's not always obvious that they're needed. To illustrate, consider the
following sequence of events: following sequence of events:
@ -570,11 +578,14 @@ following sequence of events:
B = 4; B = 4;
<write barrier> <write barrier>
WRITE_ONCE(P, &B); WRITE_ONCE(P, &B);
Q = READ_ONCE(P); Q = READ_ONCE_OLD(P);
D = *Q; D = *Q;
There's a clear data dependency here, and it would seem that by the end of the [!] READ_ONCE_OLD() corresponds to READ_ONCE() of pre-4.15 kernel, which
sequence, Q must be either &A or &B, and that: doesn't imply an address-dependency barrier.
There's a clear address dependency here, and it would seem that by the end of
the sequence, Q must be either &A or &B, and that:
(Q == &A) implies (D == 1) (Q == &A) implies (D == 1)
(Q == &B) implies (D == 4) (Q == &B) implies (D == 4)
@ -588,8 +599,8 @@ While this may seem like a failure of coherency or causality maintenance, it
isn't, and this behaviour can be observed on certain real CPUs (such as the DEC isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
Alpha). Alpha).
To deal with this, a data dependency barrier or better must be inserted To deal with this, READ_ONCE() provides an implicit address-dependency barrier
between the address load and the data load: since kernel release v4.15:
CPU 1 CPU 2 CPU 1 CPU 2
=============== =============== =============== ===============
@ -598,7 +609,7 @@ between the address load and the data load:
<write barrier> <write barrier>
WRITE_ONCE(P, &B); WRITE_ONCE(P, &B);
Q = READ_ONCE(P); Q = READ_ONCE(P);
<data dependency barrier> <implicit address-dependency barrier>
D = *Q; D = *Q;
This enforces the occurrence of one of the two implications, and prevents the This enforces the occurrence of one of the two implications, and prevents the
@ -615,13 +626,13 @@ odd-numbered bank is idle, one can see the new value of the pointer P (&B),
but the old value of the variable B (2). but the old value of the variable B (2).
A data-dependency barrier is not required to order dependent writes An address-dependency barrier is not required to order dependent writes
because the CPUs that the Linux kernel supports don't do writes because the CPUs that the Linux kernel supports don't do writes until they
until they are certain (1) that the write will actually happen, (2) are certain (1) that the write will actually happen, (2) of the location of
of the location of the write, and (3) of the value to be written. the write, and (3) of the value to be written.
But please carefully read the "CONTROL DEPENDENCIES" section and the But please carefully read the "CONTROL DEPENDENCIES" section and the
Documentation/RCU/rcu_dereference.rst file: The compiler can and does Documentation/RCU/rcu_dereference.rst file: The compiler can and does break
break dependencies in a great many highly creative ways. dependencies in a great many highly creative ways.
CPU 1 CPU 2 CPU 1 CPU 2
=============== =============== =============== ===============
@ -629,12 +640,12 @@ break dependencies in a great many highly creative ways.
B = 4; B = 4;
<write barrier> <write barrier>
WRITE_ONCE(P, &B); WRITE_ONCE(P, &B);
Q = READ_ONCE(P); Q = READ_ONCE_OLD(P);
WRITE_ONCE(*Q, 5); WRITE_ONCE(*Q, 5);
Therefore, no data-dependency barrier is required to order the read into Therefore, no address-dependency barrier is required to order the read into
Q with the store into *Q. In other words, this outcome is prohibited, Q with the store into *Q. In other words, this outcome is prohibited,
even without a data-dependency barrier: even without an implicit address-dependency barrier of modern READ_ONCE():
(Q == &B) && (B == 4) (Q == &B) && (B == 4)
@ -645,12 +656,12 @@ can be used to record rare error conditions and the like, and the CPUs'
naturally occurring ordering prevents such records from being lost. naturally occurring ordering prevents such records from being lost.
Note well that the ordering provided by a data dependency is local to Note well that the ordering provided by an address dependency is local to
the CPU containing it. See the section on "Multicopy atomicity" for the CPU containing it. See the section on "Multicopy atomicity" for
more information. more information.
The data dependency barrier is very important to the RCU system, The address-dependency barrier is very important to the RCU system,
for example. See rcu_assign_pointer() and rcu_dereference() in for example. See rcu_assign_pointer() and rcu_dereference() in
include/linux/rcupdate.h. This permits the current target of an RCU'd include/linux/rcupdate.h. This permits the current target of an RCU'd
pointer to be replaced with a new modified target, without the replacement pointer to be replaced with a new modified target, without the replacement
@ -667,20 +678,21 @@ not understand them. The purpose of this section is to help you prevent
the compiler's ignorance from breaking your code. the compiler's ignorance from breaking your code.
A load-load control dependency requires a full read memory barrier, not A load-load control dependency requires a full read memory barrier, not
simply a data dependency barrier to make it work correctly. Consider the simply an (implicit) address-dependency barrier to make it work correctly.
following bit of code: Consider the following bit of code:
q = READ_ONCE(a); q = READ_ONCE(a);
<implicit address-dependency barrier>
if (q) { if (q) {
<data dependency barrier> /* BUG: No data dependency!!! */ /* BUG: No address dependency!!! */
p = READ_ONCE(b); p = READ_ONCE(b);
} }
This will not have the desired effect because there is no actual data This will not have the desired effect because there is no actual address
dependency, but rather a control dependency that the CPU may short-circuit dependency, but rather a control dependency that the CPU may short-circuit
by attempting to predict the outcome in advance, so that other CPUs see by attempting to predict the outcome in advance, so that other CPUs see
the load from b as having happened before the load from a. In such a the load from b as having happened before the load from a. In such a case
case what's actually required is: what's actually required is:
q = READ_ONCE(a); q = READ_ONCE(a);
if (q) { if (q) {
@ -927,9 +939,9 @@ General barriers pair with each other, though they also pair with most
other types of barriers, albeit without multicopy atomicity. An acquire other types of barriers, albeit without multicopy atomicity. An acquire
barrier pairs with a release barrier, but both may also pair with other barrier pairs with a release barrier, but both may also pair with other
barriers, including of course general barriers. A write barrier pairs barriers, including of course general barriers. A write barrier pairs
with a data dependency barrier, a control dependency, an acquire barrier, with an address-dependency barrier, a control dependency, an acquire barrier,
a release barrier, a read barrier, or a general barrier. Similarly a a release barrier, a read barrier, or a general barrier. Similarly a
read barrier, control dependency, or a data dependency barrier pairs read barrier, control dependency, or an address-dependency barrier pairs
with a write barrier, an acquire barrier, a release barrier, or a with a write barrier, an acquire barrier, a release barrier, or a
general barrier: general barrier:
@ -948,7 +960,7 @@ Or:
a = 1; a = 1;
<write barrier> <write barrier>
WRITE_ONCE(b, &a); x = READ_ONCE(b); WRITE_ONCE(b, &a); x = READ_ONCE(b);
<data dependency barrier> <implicit address-dependency barrier>
y = *x; y = *x;
Or even: Or even:
@ -968,8 +980,8 @@ Basically, the read barrier always has to be there, even though it can be of
the "weaker" type. the "weaker" type.
[!] Note that the stores before the write barrier would normally be expected to [!] Note that the stores before the write barrier would normally be expected to
match the loads after the read barrier or the data dependency barrier, and vice match the loads after the read barrier or the address-dependency barrier, and
versa: vice versa:
CPU 1 CPU 2 CPU 1 CPU 2
=================== =================== =================== ===================
@ -1021,8 +1033,8 @@ STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
V V
Secondly, data dependency barriers act as partial orderings on data-dependent Secondly, address-dependency barriers act as partial orderings on address-
loads. Consider the following sequence of events: dependent loads. Consider the following sequence of events:
CPU 1 CPU 2 CPU 1 CPU 2
======================= ======================= ======================= =======================
@ -1067,8 +1079,8 @@ effectively random order, despite the write barrier issued by CPU 1:
In the above example, CPU 2 perceives that B is 7, despite the load of *C In the above example, CPU 2 perceives that B is 7, despite the load of *C
(which would be B) coming after the LOAD of C. (which would be B) coming after the LOAD of C.
If, however, a data dependency barrier were to be placed between the load of C If, however, an address-dependency barrier were to be placed between the load
and the load of *C (ie: B) on CPU 2: of C and the load of *C (ie: B) on CPU 2:
CPU 1 CPU 2 CPU 1 CPU 2
======================= ======================= ======================= =======================
@ -1078,7 +1090,7 @@ and the load of *C (ie: B) on CPU 2:
<write barrier> <write barrier>
STORE C = &B LOAD X STORE C = &B LOAD X
STORE D = 4 LOAD C (gets &B) STORE D = 4 LOAD C (gets &B)
<data dependency barrier> <address-dependency barrier>
LOAD *C (reads B) LOAD *C (reads B)
then the following will occur: then the following will occur:
@ -1101,7 +1113,7 @@ then the following will occur:
| +-------+ | | | +-------+ | |
| | X->9 |------>| | | | X->9 |------>| |
| +-------+ | | | +-------+ | |
Makes sure all effects ---> \ ddddddddddddddddd | | Makes sure all effects ---> \ aaaaaaaaaaaaaaaaa | |
prior to the store of C \ +-------+ | | prior to the store of C \ +-------+ | |
are perceptible to ----->| B->2 |------>| | are perceptible to ----->| B->2 |------>| |
subsequent loads +-------+ | | subsequent loads +-------+ | |
@ -1292,7 +1304,7 @@ Which might appear as this:
LOAD with immediate effect : : +-------+ LOAD with immediate effect : : +-------+
Placing a read barrier or a data dependency barrier just before the second Placing a read barrier or an address-dependency barrier just before the second
load: load:
CPU 1 CPU 2 CPU 1 CPU 2
@ -1816,20 +1828,20 @@ which may then reorder things however it wishes.
CPU MEMORY BARRIERS CPU MEMORY BARRIERS
------------------- -------------------
The Linux kernel has eight basic CPU memory barriers: The Linux kernel has seven basic CPU memory barriers:
TYPE MANDATORY SMP CONDITIONAL TYPE MANDATORY SMP CONDITIONAL
=============== ======================= =========================== ======================= =============== ===============
GENERAL mb() smp_mb() GENERAL mb() smp_mb()
WRITE wmb() smp_wmb() WRITE wmb() smp_wmb()
READ rmb() smp_rmb() READ rmb() smp_rmb()
DATA DEPENDENCY READ_ONCE() ADDRESS DEPENDENCY READ_ONCE()
All memory barriers except the data dependency barriers imply a compiler All memory barriers except the address-dependency barriers imply a compiler
barrier. Data dependencies do not impose any additional compiler ordering. barrier. Address dependencies do not impose any additional compiler ordering.
Aside: In the case of data dependencies, the compiler would be expected Aside: In the case of address dependencies, the compiler would be expected
to issue the loads in the correct order (eg. `a[b]` would have to load to issue the loads in the correct order (eg. `a[b]` would have to load
the value of b before loading a[b]), however there is no guarantee in the value of b before loading a[b]), however there is no guarantee in
the C specification that the compiler may not speculate the value of b the C specification that the compiler may not speculate the value of b
@ -2749,7 +2761,8 @@ is discarded from the CPU's cache and reloaded. To deal with this, the
appropriate part of the kernel must invalidate the overlapping bits of the appropriate part of the kernel must invalidate the overlapping bits of the
cache on each CPU. cache on each CPU.
See Documentation/core-api/cachetlb.rst for more information on cache management. See Documentation/core-api/cachetlb.rst for more information on cache
management.
CACHE COHERENCY VS MMIO CACHE COHERENCY VS MMIO
@ -2889,8 +2902,8 @@ AND THEN THERE'S THE ALPHA
The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that, The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
some versions of the Alpha CPU have a split data cache, permitting them to have some versions of the Alpha CPU have a split data cache, permitting them to have
two semantically-related cache lines updated at separate times. This is where two semantically-related cache lines updated at separate times. This is where
the data dependency barrier really becomes necessary as this synchronises both the address-dependency barrier really becomes necessary as this synchronises
caches with the memory coherence system, thus making it seem like pointer both caches with the memory coherence system, thus making it seem like pointer
changes vs new data occur in the right order. changes vs new data occur in the right order.
The Alpha defines the Linux kernel's memory model, although as of v4.15 The Alpha defines the Linux kernel's memory model, although as of v4.15

37
tools/memory-model/Documentation/litmus-tests.txt
View file

@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model (LKMM) include:
carrying a dependency, then the compiler can break that dependency carrying a dependency, then the compiler can break that dependency
by substituting a constant of that value. by substituting a constant of that value.
Conversely, LKMM sometimes doesn't recognize that a particular Conversely, LKMM will sometimes overestimate the amount of
optimization is not allowed, and as a result, thinks that a reordering compilers and CPUs can carry out, leading it to miss
dependency is not present (because the optimization would break it). some pretty obvious cases of ordering. A simple example is:
The memory model misses some pretty obvious control dependencies
because of this limitation. A simple example is:
r1 = READ_ONCE(x); r1 = READ_ONCE(x);
if (r1 == 0) if (r1 == 0)
smp_mb(); smp_mb();
WRITE_ONCE(y, 1); WRITE_ONCE(y, 1);
There is a control dependency from the READ_ONCE to the WRITE_ONCE, The WRITE_ONCE() does not depend on the READ_ONCE(), and as a
even when r1 is nonzero, but LKMM doesn't realize this and thinks result, LKMM does not claim ordering. However, even though no
that the write may execute before the read if r1 != 0. (Yes, that dependency is present, the WRITE_ONCE() will not be executed before
doesn't make sense if you think about it, but the memory model's the READ_ONCE(). There are two reasons for this:
intelligence is limited.)
The presence of the smp_mb() in one of the branches
prevents the compiler from moving the WRITE_ONCE()
up before the "if" statement, since the compiler has
to assume that r1 will sometimes be 0 (but see the
comment below);
CPUs do not execute stores before po-earlier conditional
branches, even in cases where the store occurs after the
two arms of the branch have recombined.
It is clear that it is not dangerous in the slightest for LKMM to
make weaker guarantees than architectures. In fact, it is
desirable, as it gives compilers room for making optimizations.
For instance, suppose that a 0 value in r1 would trigger undefined
behavior elsewhere. Then a clever compiler might deduce that r1
can never be 0 in the if condition. As a result, said clever
compiler might deem it safe to optimize away the smp_mb(),
eliminating the branch and any ordering an architecture would
guarantee otherwise.
2. Multiple access sizes for a single variable are not supported, 2. Multiple access sizes for a single variable are not supported,
and neither are misaligned or partially overlapping accesses. and neither are misaligned or partially overlapping accesses.